In release 6 of the WCDMA (Wideband Code Division Multiple Access) specification, a High Speed Uplink Packet Access (HSUPA) (also called Enhanced Uplink) communication scheme is defined in addition to the downlink High Speed Data Packet Access (HSDPA) scheme in order to match the bit rates provided by the latter, so as to cater for improved interactive, background and streaming services. In prior art document 3GPP TS 25.309, “FDD Enhanced Uplink; Overall Description; Stage 2”, Version V6.6.0 of 2006-04-06 the Enhanced UL (Uplink) is described.
Relevant sections can moreover be found in 3GPP References TS25.211 and TS25.321.
In FIG. 1, a HSUPA network overview (HSDPA related channels are not included in the figure) is indicated. The network comprises a Core Network communicating with a Radio Network Controller (RNC, S-RNC, B-RNC) over the lu interface, or lur interface; a first base station, Node B, B1, a second base station, Node B, B2, both base stations comprising an EUL scheduler unit. The EUL Scheduler (EUL_SCH) is also denoted the MAC-e Scheduler, and communicating with the RNC over respective lub interfaces.
The following HSUPA channels are transmitted over the air interface; the E-AGCH to convey absolute grant signalling from the MAC-e scheduler towards the UEs, the E-RGCH for relative grant signalling, HICH to convey acknowledgement feedback from Node-B decoding of UE transmitted data, Dedicated Physical Channel (DPCH) or Fractional DPCH to convey Transmit Power Control (TPC) commands, Enhanced DPDCH (E-DPDCH) to convey the MAC-e payload and Enhanced DPCCH (E-DPCCH) to convey the control signalling of the MAC-e.
Node B1 corresponds to the serving cell in this example (E-AGCH is only transmitted from the serving cell) and node B2 corresponds to a non-serving cell.
Document 3GPP TS 25.309 FDD, Enhanced Uplink Overall description, mentioned above gives an overview of the Enhanced Uplink functionality.
In prior art document “High Speed Uplink Packet Access (HSUPA); White Paper, application note 1MA94”, Rohde Schwarz, 01.2006, an overview of the HSUPA can also be found.
According to the HSUPA specification, the Enhanced Dedicated Channel (E-DCH) high speed uplink transport channel offers a number of new features such as: short Transmission Time Interval (TTI), Fast Hybrid Automatic Repeat Request (ARQ) with soft recombining, fast scheduling for reduced delays, increased data rates and increased capacity.
When a UE is setting up communication with a Node B, the setup procedure may be followed by a HSDPA session, for e.g. downloading/surfing an internet page using TCP. Depending on the capabilities of the user entity, this may moreover involve HSUPA transmissions whereby Node B that transmits TCP messages on the HSDPA downlink channel will receive TCP acknowledgements on the E-DCH uplink to Node B. Since Node-B determines, or schedules, at which pace a UE shall transmit on E-DCH, Node-B utilises the E-AGCH to convey its scheduling decisions. A shorter delay, measured from the time until a TCP data segment is sent downlink until a TCP acknowledgement as a response is sent uplink, leads to a decreased downloading time of file transfers etc, due to the shorter round trip time estimate of the TCP layer.
During the procedure upon which the user entity becomes ready to use a HSUPA service with Node B, the user entity is informed about which E-AGCH code it is supposed to receive downlink traffic on. For this purpose, the E-AGCH, which is a shared channel within the cell, is used via the RRC (Radio Resource Control) protocol.
E-AGCH channels are configured to a Node B in a configuration or re-configuration procedure with the RNC via the NBAP (Node B Application Part) signalling protocol.
The Node B MAC-e Scheduler issues “absolute grants” on the downlink E-AGCH channel, that is, messages which grant the user entity the right to transmit at given bit rates on the uplink. Since bandwidth needs vary dynamically over time, it is beneficial that the power emissions by user entities are regulated speedily so that bandwidth is not unnecessarily wasted.
The E-AGCH can be defined to have a number of one to several channelization codes (presently, up to four).
A problem in the serving Node-B is that is does not have complete information about the UEs for whom it is responsible for scheduling transmission rates. This lack of knowledge in the serving Node-B may lead to a non-optimal utilization of resources both in the Node-B and on the air.
In order to illustrate the above problem, consider a case with a UE, a serving Node-B, and a non-serving Node-B as shown in FIG. 1 and the handshake diagram of FIG. 3. Note that a UE can be connected to multiple non-serving Node-B in the general case. Consider further that the end user of this UE starts to upload information to the internet, e.g. photo images to a data repository.
The UE starts by transmitting a scheduling request 1 on E-DCH via the SI field or E-DPCCH via the happy bit (set to not happy) to serving Node B. Node B transmits an absolute grant 2 on E-AGCH allowing the UE to transmit at 32 kbps. The UE starts by transmitting data 3 at the given rate but signals not happy 3 in the outband E-DCH control signalling fields in order to increase the data rate.
The serving Node B transmits a new E-AGCH 4 allowing the UE to transmit at 128 kbps 5. However, the UE is still not happy concerning the granted data rate and signals this in the message 5. Subsequently, the serving node B grants 470 kbps to the UE, 6, and the UE transmits at this data rate 7.
At this stage, the non-serving Node B considers that the interference situation is too high and transmits a Broadcast Overload Indication 8, i.e. a signal on the E-RGCH channel with value Down.
The UE receives the Down signal 8 and reduces 9 its rate to 128 kbps. Serving Node B will now detect that the UE signals 9 “not happy” but that the UE seems to transmit at a lower rate than allowed to.
The following problem now appears for the serving Node B.:    1) Has the UE received a Down from a non-serving Node B, and is that the reason why serving Node-B detects a receiving decoded rate lower than the granted rate?    2) Has the UE failed to decode the previous E-AGCH allowing a rate of 470 kbps?    3) Is the lower rate a problem related to e.g. temporarily power issues of the UE?
Regardless whether Node B assumes that case 1, 2 or 3 above has occurred; Node B needs to send out a new absolute grant sooner or later. It shall not wait too long since case 2 above could be the reason why the UE is still unhappy while sending on 128 kbps. If case 1 has happened, the interference situation may also improve quickly.
Hence, after step 9, the serving node B sends out a new absolute grant that allows the UE to transmit at 470 kbps 10, which is followed by the UE, 11.
For the non-serving cell, 128 kbps is the limit and therefore it transmits a down 12. UE regulates its rate to 128 kbps 13 and indicates “Not happy”.
As can be seen, the Broadcast Overload Indication 8 causes a toggling situation where the UE rate is increased-decreased-increased which may cause serious interference in all non-serving cells or any cell in the vicinity of the serving cell.